fix edge color format in graphviz_string

[vdelecroix]

graphviz color format should be one of the following

• HSV triple such as ".08396, 0.4862,0.8549"
• RGB triple such as #DA70D6
• a color name such as orchid

As a consequence the following is broken

sage: G = Graph([(0,1)])
sage: G.set_latex_options(edge_colors={(0,1): (0.3, 0.9, 0.0)}, format='dot2tex')
sage: view(G)

To make it coherent with matplotlib option as in

sage: plot(x, color=(0.3, 0.9, 0))

we convert 3-tuples in RGB format using the to_hex function from matplotlib.

[vdelecroix]

New commits:

​b4176d5	dirty fix for edge colors with dot2tex
​334b48a	fix graphviz string color handling

[slabbe]

Why do you do this?

+to_hex = None   # overriden when needed in graphviz_string

to gain some time at start up?

sage: %time from matplotlib.colors import to_hex
CPU times: user 60 ms, sys: 12 ms, total: 72 ms
Wall time: 70.7 ms

Then why not lazy import it?

[git]

Branch pushed to git repo; I updated commit sha1. New commits:

​e72b93e

use lazy import for to_hex

[vdelecroix]

Good suggestion! Thanks.

[slabbe]

Adding format='dot2tex' in the description otherwise view(G) just works and does not illustrate the problem.

[slabbe]

sage -tp --optional=dot2tex,sage,graphviz --show-skipped src/sage/graphs/generic_graph.py

looks good.

I added a very small change. If you agree, change this ticket to positive review.

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New commits:

​e3e3ff4

25121: keep_alpha=False (Explicit is better than implicit.)

[vdelecroix]

Merci Sébastien!